Everything in the universe is made of atoms. An atom is the smallest
amount you can have of any element. An atom is composed of a positively
charged nucleus made of protons and neutrons, surrounded by negatively
charged electrons.

When atoms become connected or bonded with other atoms, they form
molecules. Molecules can be as simple as a molecule of hydrogen
gas, which is made of nothing but two hydrogen atoms. Molecules
can also be made up of thousands of atoms of many different elements.
Many biological molecules, including proteins and DNA, are large
and complex.

The bonds between the atoms in a molecule can store and release
energy as the molecules are made or broken apart. This can happen
because different molecular bonds can have different energy states.
It takes energy to create a bond with a high energy state. This
happens when green plants make carbohydrates during photosynthesis.
When such bonds are broken, energy is released, as when carbohydrates
are oxydized during respiration.